1. Law of Universal Gravitation
Chapter 12
November 9/10

2. Objectives Analyze how Earth is falling
State Newton’s law of universal gravitation
Evaluate the significance of inverse-square law

3. Introduction All objects have gravity
Therefore, all objects attract one another

4. The Falling Apple An apple falls to Earth because of gravity
Does Earth’s gravity pull the moon?
YES
Why doesn’t the moon fall into Earth?
It does but it doesn’t crash because of tangential speed

5. The Falling Moon Earth’s gravity pulls moon = centripetal force
Moon falls beneath the straight line it would follow if no force acted on it
Below the tangential speed line
Figure 9.2
Confirmed with experiment
Figure 9.4

6. The Falling Earth Earth (and other planets) falls around the sun
Why don’t planets crash into sun?
Because of tangential velocities
What would happen if tangential speeds went away?
Crash into sun
This has happened to asteroids and other matter without a big enough tangential speed

7. Check for understanding
In the picture below use vectors and words to describe how the Earth orbits the sun
Tangential speed
Sun
Earth
Fc = gravity
Earth stays in orbit as the resultant of the sun’s gravity and Earth’s tangential speed

8. Newton’s Law of Universal Gravitation
Newton discovered gravity is universal
Everything pulls on everything else
Law of Universal Gravitation
“Every object attracts every other object with a force that for any two objects is directly proportional to the mass of each object”
Greater masses = greater force of attraction
Greater distance (farther away) = less force of attraction

9. Newton’s Law of Universal Gravitation (con’t)
Law can be expressed as an equation
FG = force of gravity (N)
G = universal constant
= 6.7 x Nm2
kg2
m1 = mass of one object (kg)
m2 = mass of other object (kg)
r = radius (m)

10. Class work- Honors
Use the equation that relates to the law of universal gravitation to solve the problem

11. Inverse Square Law
When a quantity varies as the inverse square of its distance from its source
Figure 9.4 and 9.5
Force of gravity depends on distance from Earth.

12. Closing
An apple falls into Earth. Why doesn’t the moon fall and crash into Earth?
It does but it doesn’t crash because of tangential speed
AND
Force of gravity is weaker since moon is farther from Earth’s surface than apple

13. Homework Next class we will: The following class is the unit exam on:
Take a quiz on all units and symbols- STUDY!
review for the unit exam
The following class is the unit exam on:
Newton’s 3 laws
Centripetal force and tangential speed
Law of universal gravitation